1. Field of the Invention
The present invention relates to a wrist mechanism of an industrial robot comprising three wrist elements rotatably supported by an arm of the robot relative to each other around respective rotation axes which are mutually inclined, which leaves a hollow space inside thereof free for passage of supply cables and pipes which are to be connected to a tool being secured to the robot wrist.
2. Description of the Related Art
Such wrist mechanisms of the industrial robots each comprising two or three wrist elements which leaves a hollow space inside thereof free for passage of supply cables and pipes, are disclosed, for instance, in U.S. Pat. Nos. 5,549,016; 6,014,909 and 5,816,108. These patents include wrist mechanisms comprising hollow drive tubes and hollow bevel gears connected thereto to transmit rotational movements by the hollow drive tubes to each of the wrist elements and which leaves a hollow space inside thereof free for passage of supply cables and pipes.
In order to pass the supply cables and pipes inside the interiors of the robot wrist, usually chains of gears are used to transmit rotational movements by the hollow drive tubes to each of the wrist elements. Especially, in cases where chains of gears are used as the final input transmitting members to each of the wrist elements, a difficulty arises in that the controllings of the positionings of the wrist elements become unstable due to backlashes in the gears. Further, the positioning becomes more unstable due to flank wears of the gears suffering from high torques applied thereon. Another difficulty arises in that the controllings of the positionings of the wrist elements from a large amount of a reflected or followed rotational movement of the third wrist element caused by the rotational movements of the first and second wrist elements, that is, since the first, second and third hollow drive tubes are independently supported by the arm of the robot, and drive the first, second and third wrist elements relative to each other, a rotational movement of the first wrist element effects or follows those of the second and third wrist elements, as well as a rotational movement of the second wrist element effects or follows that of the third wrist element.
An object of the present invention is to provide a wrist mechanism of an industrial robot which has a sufficient hollow space inside thereof free for passage of supply cables and pipes which are to be connected to a tool being secured to the tool flange, wherein difficulties in that the controllings of the positionings of wrist elements become unstable due to backlashes and flank wears in the gears are eliminated and an accuracies of controllings thereof are increased.
Another object of the present invention is to provide a wrist mechanism of an industrial robot wherein amounts of reflected rotational movements of the second and/or third wrist elements affected by the rotational movements of the first and second wrist elements are made small, thereby the controllings of the positionings of the wrist elements become easy and an accuracies of controllings thereof are increased.
These and other objects are achieved by a wrist mechanism of an industrial robot comprising: first, second and third hollow drive tubes rotatably supported by an arm of the robot relative to each other coaxially around a first axis; a first wrist element supported by the first drive tube rotatably around the first axis; a first ring-shaped hollow reduction unit carried by the first wrist element, an input shaft of the first reduction unit being drivingly connected to the second drive tube via a first pair of bevel gears; a second wrist element supported by an output shaft of the first reduction unit rotatably around a second axis inclined with respect to the first axis; a second ring-shaped hollow reduction unit carried by the second wrist element, an input shaft of the second reduction unit being drivingly connected to the third drive tube via second and third pairs of bevel gears, the second and third pairs of bevel gears being connected by a tubular drive shaft; and, a third wrist element or tool flange carried by an output shaft of the second reduction unit rotatably around a third axis inclined with respect to the second axis; said third drive tube, the tubular drive shaft and the second reduction unit leaving a sufficient hollow space inside thereof free for passage of supply cables and pipes which are to be connected to a tool being secured to the tool flange.
By such an arrangements, by using ring-shaped hollow reduction units in the wrist mechanism, a sufficient hollow space is secured inside thereof free for passage of supply cables and pipes which are to be connected to a tool being secured to the tool flange. Further, since the amount of backlashes and flank wears in the gears are reduced within a fractions of the reduction ratios of each of the reduction units, the difficulties in that the controllings of the positionings of the wrist elements become unstable due to backlash and flank wears in the gears are eliminated and an accuracies of controllings thereof are increased. Similarly, since amounts of reflected rotational movements of the second and/or third wrist elements affected by the rotational movements of the first and second wrist elements are reduced within fractions of the reduction ratios of each of the reduction units, they resulted that the amounts of the reflected rotational movements are made so small, thereby the controllings of the positionings of the wrist elements become easy and an accuracies of controllings thereof are increased.